Manufacture of white lead.



No. 771,024. I PATENIED SEPT. 27, 1904. A J. OETTLI.

MANUFACTURE OF WHITE LEAD.

APPLIOATION FILED JULY 6, 1903.

N0 MODEL.

WITNESSES: 1 INIVENTOR A framers UNITED STATES Patented September 27,1904.

PATENT OFFICE.

JACQUES OETTLI, OF LAUSANNE, SIVITZERLAND, ASSIGNOR TO THE FIRM OFSYNDICAT POUR LEXPLOITATION DES INVENTIONS DU PROFES- SEUR OETTLI, OFBERNE, SWITZERLAND.

MANUFACTURE OF WHITE LEAD.

SPECIFICATION forming part of Letters Patent No. 771,024, datedSeptember 27, 1904;. Application filed July 6, 1903. Serial No. 164,265.(No specimens.)

To all whom it may concern:

while, moreover, the product obtained according to this invention isabsolutely pure and of a very low price.

By means of the processes heretofore employed white lead may be producedin a powdery state, quite dry, and impalpable, and this product isliable to exert unpleasant and poisonous effects upon the workmenmanufacturing or making use of such product.

In this improved process, which has the great advantage of avoiding theaforesaid inconveniences, white lead is obtained by means of leadelectrodes and an electric current of relatively weak voltage, anelectrolyte formed of a one-per-cent. solution of common salt and bycarbonating the product of the electrolysis after having separated ittherefrom. The white lead, absolutely pure, settles down in the form ofa paste in the middle of the electro-, lyte if it has been carbonated inthe bath itself. It is then extracted and introduced into a paint millor grinder adapted to mix it with oil in order that it may be kept inthe condition of a paste ready for sale.

It is important in the manufacture of White lead by means of thisprocess to avoid the accidental formation of superior oxids of lead andby secondary reaction of oxychlorid of lead.

In order to obtain the desired result, it is necessary to observe thefollowing conditions: The electrolyte is a one-per-cent. solution ofcommon salt. This quantity must not be exceeded,'experienee havingproved that a more concentrated solution gives rise to other compounds.The products of the electrolysis and of the secondary reactions areconstantly the hydroxid of lead, (PbOH2,) and the chlorid of sodium, (NaCL) which is incessantly regenerated. Therefore the same electrolytemay always be used again, but mixed with water in order to obtain theabove-mentioned degree of concentration in proportion to the absorption.of the water by the chemical reactions. The water and lead are theonlyraw material used by the electrolysis. The hydrate of lead is thencarbonated by the direct introduction of carbonic acid into the bath.The carbonic acid, or carbon which produces it, is the third rawmaterial. There is-no other, because the carbonation finishes theproduct in accordance with the following equations:

The electrodes are formed of plates of lead the submerged surfaces ofwhich are of a size depending on the intensity of current.

A too great electrical density produces the formation of compounds oflead resulting from the oxidizing action of the chlorid-as, forinstance, peroxid of lead, (PbO2,)-and through them several oxychloridsof lead.

The density which seems to be the most suitable is that of 1 amp. dm Itmust in no case exceed 2 amp. dm

Voltage: The chlorid of sodium must be decomposed. Its heat ofcombination is 96.2 cal, wherefrom is calculated the voltage :4, 2volts, and on account of the exterior resistances, which may be easilyreduced to a minimum, it amounts to five volts. Therefore there must befive volts in every tank whatever its size may be. It is preferable tomake use of a series of ten tanks corresponding to a current of fiftyvolts, which seems to give satisfactory results. As to the water used inthe tanks, the softer it is the better, and rain' water or the water ofa lake or ariver is preferable. The temperature of the electrolyteshould not exceed that of the ambient air say about 15 O. A highertemperature would probably lead to the formation of superior oXids oflead and by secondary reaction of oxychlorid of lead, and this of coursemust be avoided. The aforesaid temperature may be easily maintained bymeans of a suitable circulation of the liquid. A distributing-tankcauses the electrolyte to flow into the tanks from which the liquid isextracted in order that the white lead which it contains may beseparated from it and to cause it to again circulate in the tanks bysending it back to the distributing-tank.

The following equations express the chemical reactions which aresimultaneously accomplished. The tanks never contain any free chlorin,as has been ascertained by means of the reaction of the liquid onstarched paper with potassium iodid.

The chlorin of lead which is produced at the anode is transformed at themoment of its formation and under theinfiuence of the soda which isproduced at the same time at the cathode into hydrate of lead, whichfalls to the bottom of the tank. The production is very rapid, and adeposit of hydrate of lead is formed on the anode and causes itspolarization. In order to prevent this deposit and at the same time touse both electrodes. the current is reversed at very shortintervalsevery five or ten minutes. Experience has shown that thismethod gives very good results both as regards the quality and thequantity of the product obtained.

The current used is a continuous current not exceeding one ampere persquare decimeter. For the purpose of preventing polarization the currentis reversed at definite intervals by means of a suitable device.

The equations show that an evolution of hydrogen takes place, andas thequantity of this gas generated is considerable a very wellventilatedplace for the operation should be provided.

It is preferable in order to prevent the liquid becoming heated to causeit to circulate. It may be collected in tanks in which one introducescarbonic acid, and after filtration it is caused to circulate again bymeans of a pump in the distributing-tank, from which it flows into thetanks.

The white lead is of better quality if the carbonation takes place justat that moment when the hydrate arises. Therefore it is preferable toeffect this operation in the tanks of the electrolyte, on account of thehydroxid of lead being a base which easily absorbs carbonic acid. Thismanner of proceeding has the advantage of preventing the formation ofsuperior oxids of lead and oxychlorids, which may also be the result ofa too long absorption of hydrate of lead in a solution of common salt.

The introduction of carbonic acid into the tanks must be regulated, andthe quantity of this gas must also be controlled. This controlling isnot difficult. The hydrated oxid of lead is a base. Therefore the liquidof the bath becomes alkaline from the beginning of the electrolysis,turmeric paper becomes brown, and the red of litmus turns blue. Carbonicacid is introduced until the basicity of the liquid begins to disappear.The neutralization of hydroxid must, however, not be complete. Thecarbonation continues by itself during the following operations by meansof the carbonic acid contained in the air.

The white lead settles down very quickly at the bottom of the tanks. Theliquid is then removed by decantation, by sucking up by means of asiphon, or otherwise and is then caused to circulate again to be usedfor the electrolysis. The white lead is then thoroughly washed andseparated from the remaining water by means of a filter-press or acentrifugal apparatus. The white lead obtained in this manner remainswet until it is prepared for sale. Consequently the dust, which is veryinjurious to health, is thus avoided. Drying should be done very slowly,the paste being first exposed to the open air, stretched on a largesurface, and is then finished in drying-rooms by means of hot air havinga temperature not exceeding 50 centigrade. For this purpose the lostheat resulting from the manufacture of carbonic acid by the combustionof the carbon may be employed.

The white lead is generally sold in the state of a paste-that is to say,ground with eight to ten per cent. of purified linseed-oil. The qualityof white lead not only depends upon the process of manufacture, but alsoupon the quality of the lead employed. Badly-refined lead containingiron, copper, arsenic, antimony, or silver produce white lead of aninferior quality. Contaminations of copper are especially to be avoided,as paints made with white lead containing copper become very quicklyyellowish. It is therefore preferable to use lead of first quality,which may be ascertained by chemical analysis.

If a density of 1 amp. dm is made use of, there must be for a current of1,000 amps. a surface of 1,000 dm 10 m per tank. Therefore the leadcannot be used in the shape of piglead, but in plates the thickness ofwhich, on account of the strong density of the lead, cannot beconsiderable. Assuming the plates to be 1 m there must be ten per tank,and if such a plate has a thickness of 1 cm. it will weigh 115 kg, whichwill give for one tank more than a ton of lead eleven and one-half tonsfor ten tanks-and working with a current of &0 KW. said plates will beworn out in ten days; but practically they must be replaced in eightdays, because they will become perforated at many points, which willinfluence the density of the current,

The worn-out plates are melted with new pig-lead and cast into plates of1 cm. thickness. In order to facilitate manufacture, it would bepreferable to use thinner plates, and in this case there would be anadvantage in laminating the lead.

.The tanks are preferably of wood, rendered water-tight and verycarefully tarred to .prevent the liquid from penetrating into the wood,any extraction from which, especially in new tanks, would injure thewhite lead, and it is therefore preferable to have them lined with lead.

The accompanying drawing shows, by way of example, a diagram of suitableapparatus embodying this invention.

In the drawing the said apparatus shows three series of fourelectrolytic tanks each; but in practice the said tanks are preferablygrouped in three series of ten tanks, which are arranged in such amanner that two series work together and one series is out of use.

a is the electrolyte-distributing tank, containing a one-per-cent.solution of common salt, and 6 represents the electrolytic tanks.

0 is a delivery-pipe provided above every tank 6 with a branch pipe 0,having a suitable tap. Each tank I) is provided with a siphon 6, adaptedto suck up from the bottom of the tank all the liquid containing thewhite lead and to conduct it into a channel d, opening into ahydro-extractor e, the shaft f of which is rotated by a suitable motor.The electrolyte flowing into the tank 6' of the hydro-extractor thenruns into the tank g, from which a pump it, actuated by a suitable mo-,tor, sucks it up through the pipe c' and conducts it back through a pipeiv to the distributing-tank a.

The lead electrodes Z are preferably formed of plates of a thickness of1 cm. and of 1 cm of surface. They are connected together and with theelectric circuit, as shown in the drawing, and said circuit contains, inthe case of a distribution with continuous current, areversing-commutator of any desired kind.

The white lead is collected in the condition of a wet paste in thehydro-extractor e and is then introduced into a paint mill or grinder,where it is mixed with oil, being then ready to be packed for sale.

I claim The process herein described of making white lead, whichconsists in electrically dissolving a lead electrode by a continuouscurrent not exceeding one ampere per square decimeter, reversed atdefinite intervals, in an electrolyte formed of a solution of commonsalt-of one per cent. and of a temperature of about 15 centigrade.

In testimony that I claim the foregoing as my invention I have signed myname in presence of two subscribing witnesses.

- JACQUES OETTLI.

